Intelligent transportation systems (ITS) are emerging for improving road safety by broadcasting real-time information regarding vehicles in the area. According to some ITS implementations, vehicles set-up a vehicular ad-hoc network (VANET) based on IEEE 802.11p dedicated short-range communications (DSRC) to exchange position, speed, road condition, or event/accident information between nearby vehicles. In some applications, such as implementations according to the European Telecommunications Standards Institute (ETSI) ITS standards, vehicles periodically broadcast common awareness messages (CAMs) at a rate of 10 Hz to report information such as current speed and position in the control channel.
In many situations, however, the continual broadcast of CAMs and other messages at this high message rate by large number of vehicles in the area will lead to a congested wireless communications medium. In particular, with a large amount of data traffic, the interference on the medium will rise and, consequently, so will the error rate in received messages and the channel access delay due to a higher frequency in raising the medium busy flag. The relevant standards suggest that in areas of high congestion, the participant vehicles should reduce the rate of CAM and other message broadcast below 10 Hz or reduce transmission power to free up the medium.